The invention relates generally to communication transmitters requiring amplitude modulation. Modern communication systems, such as cable, cellular and satellite communications, employ non-constant envelop digital modulation to increase spectral efficiency, such as quadrature phase-shift keying (QPSK), offset QPSK (OQPSK) and quadrature amplitude modulation (QAM) etc. To transmit a non-constant envelop signal, a linear power amplifier is required in the transmitter which usually has a low power efficiency and insufficient linearity. A direct digital amplitude modulator is of great interest for a transmitter to achieve simple structure, good linearity and high power efficiency in various communication systems.
FIG. 1 and FIG. 2 are the block diagrams of an exemplary QAM modulator. In FIG. 1, input data 100 is split into digital in-phase (I) signal 102a and quadrature (Q) signal 102b through digital signal processing (DSP) unit 101. Digital-to-analog converters (DACs) 103a and 103b are used to convert I signal 102a and Q signal 102b to analog signals, and low pass filters 104a and 104b are used to clear the alias components of the DAC outputs. In amplitude modulators 105a and 105b, the outputs of low pass filters 104a and 104b modulate carrier signals 106a and 106b which phases are separated in 90 degrees. The outputs of modulators 105a and 105b are combined in combiner 107, and the output of combiner 107 is amplified through linear power amplifier 108.
The polar modulation technique is shown in FIG. 2. Data 200 is split into digital amplitude signal 202a and phase signal 202b by DSP unit 201. Digital amplitude signal 202a is converted to analog amplitude signal through DAC 203, and low pass filter 205 clears the alias components of the DAC output. Digital phase signal 202b modulates input carrier 207 in phase modulator 204, and modulated carrier signal 206b is amplified through power amplifier 208. The gain of power amplifier 208 is linearly controlled by analog amplitude signal 206a. The embodiment in FIG. 2 is more power efficient than that in FIG. 1, as the modulated carrier signal is a constant envelop signal, a nonlinear power amplifier can be used.
FIG. 3a shows a conventional DAC 301 with an analog reconstruction filter 302, and the spectra of digital input signal 300 and analog output signal 303 are showed in FIG. 3b and FIG. 3c respectively. From FIGS. 3b and 3c, when a broadband signal is transmitted, it is difficult to filter out the alias signals of the DAC output, and the wanted signal is distorted due to the DAC's non-flat sinc response. As the clock frequency (fs) increases, the attenuation of the alias signal is increased, and the distortion of the signal is reduced.
The embodiments in FIG. 1 and FIG. 2 are not suitable for full system integration, especially in broadband data transmission. Due to the limitation of clock frequency (fs), they need complex analog reconstruction filters to remove alias signals. The analog reconstruction filter may even need discrete components.
In mobile communications, a high efficiency power amplifier (PA) is essential, because the PA dominates the power consumption of the portable system. RF power amplifiers are most efficient when they work in switching mode and amplify a constant envelop signal. The amplification of a non-constant envelop signal requires a linear PA which is inherently less power efficient.